How do Captains get promoted to Warchief?

Another simple and fast way (which is also non-lethal) to promote Uruks rapidly is to go to a Forge tower and Advance Time over and over again. Eventually, some random captain will take the place of the deceased Warchief, and voila, you can now brand him, or kill him if you want an epic rune (but you can also use advance time further to get other members of the Nemesis system over power level 10 to get an epic rune, regardless of rank.)

It can also be used to level up Uruk captains over power level 25 and get the achievement associated with that action. Although, by that point, he probably would have virtually no weaknesses so you would need to interrogate a normal Uruk instead of a Worm or a Captain in order to have an easier time beating him.

I advanced time over and over and over again and eventually, I had a new set of Warchiefs in Udün and power level 14-19 captains serving power level 9 warchiefs. Additionally, it's interesting to see the dynamics that take place each time when you do advance time, for example, a level 20 captain facing down a Graug and succeeding, a level 17 and a level 16 dueling for supremacy, and a level 22 having a feast for his followers, only to be ambushed by the pathetic level 6 newcomer. It's really fun to let the Nemesis system take its course. All Talion has to do is stay in stealth and let the game do its own thing.

Additionally, there was one Uruk that killed me maybe 16 times, and was guarding a level 6 warchief. He was level 23. He lost a beast hunt with a Graug and my "Nemesis" is no more. Branding that warchief was the easiest thing ever, since he had no others, and I used wraith flash to scare off the other normal Uruk.